Marine acoustic velocity profiling

ABSTRACT

A marine vessel streams a plurality of hydrophones while sequentially generating acoustic waves along a marine traverse at spaced-apart intervals to provide common depth point coverage of reflecting horizons within the water layer. The hydrophones receive reflections from the water layer to generate signals which are then recorded. Cross-correlations are taken of selected gates of the signals, the gates being related to one another in dependence upon the horizontal spacing between ones of the hydrophones. Acoustic velocity is the variable in the crosscorrelations. The cross-correlation products are summed to produce a composite velocity correlation function, the peak point of which provides an indication of the acoustic velocity of the underwater section through which the reflections of the acoustic waves travel. Arithmetic operations are performed on the acoustic velocities to provide a substantially real-time profile of the acoustic velocity of the intervals between the underwater reflecting horizons.

United States Patent Feb. 22, 1972 Burg [541 MARINE ACOUSTIC VELOCITYPROFILING [72] Inventor: Kenneth E. Burg, Dallas, Tex.

[73] Assignee: Texas Instruments Incorporated, Dallas,

Tex.

[22] Filed: Aug. 28, 1969 [21] Appl. No.: 853,646

[52] US. Cl .340, 340/155 [51] Int. Cl. [58] FieldolSearch ..340/l5.5,7

[56] References Cited UNITED STATES PATENTS 3,274,541 9/1966 Embree..340/l5.5 3,284,763 11/1966 Burg et all ....340l15.5 3,292,141 12/1966Hines et a1... 3,300,754 1/1967 Lee et al 3,317,890 5/1967 Hensley, Jr.3,409,871 11/1968 Heffring ..340/15.5 3,417,370 12/1968 Brey ..340/l5.5

Primary Examiner-Rodney D. Bennett, Jr.

Assistant Examiner-N. Moskowitz Attorney-James 0. Dixon, Andrew M.Hassell, Harold Levine, Rene E. Grossman and Melvin Sharp ABSTRACT Amarine vessel streams a plurality of hydrophones while sequentiallygenerating acoustic waves along a marine traverse at spaced-apartintervals to provide common depth point coverage of reflecting horizonswithin the water layer. The hydrophones receive reflections from thewater layer to generate signals which are then recorded.Cross-correlations are taken of selected gates of the signals, the gatesbeing related to one another in dependence upon the horizontal spacingbetween ones of the hydrophones. Acoustic velocity is the variable inthe cross-correlations. The cross-correlation products are summed toproduce a composite velocity correlation function, the peak point ofwhich provides an indication of the acoustic velocity of the underwatersection through which the reflections of the acoustic waves travel.Arithmetic operations are performed on the acoustic velocities toprovide a substantially real-time profile of the acoustic velocity ofthe intervals between the underwater reflecting horizons.

6 Claims, 3 Drawing Figures 5 17 Y a i i lg 14 t 5..

PATENTEDFEB22 I972 3. 644.882

sum 1 or 2 AT T-V NMO CROSS CORRELATE 5 INVENTOR KENNETH E. BURG FIG. I

PATENTEDFEBZZ I972 3.644.882

SHEEI 2 0F 2 I 20 AcouSTIc SouRcE 2 TI V T2 SI 3 T3 24 I E HYDROPHONEr28 VS'Z SYSTEM r V V I I V V Y Y I 7 TIME vARIED j FIG 3 GAIN ANDSWITCHING DEvIcE 7 a2 SUMMING SYSTEM J 84 TIME vARIED MULTIPLEXER ADAcoNvERTER NAVIGATION ENGINE AND STORAGE STEERING SYSTEM CONTROLSCOMPUTER 96 94 LSToRAsE P LDISPLAY P UNDER SEA WARFARE SYSTEM INvENTDRKENNETH E. BURG MARINE ACOUSTIC VELOCITY PROFILING This inventionrelates to velocity profiling, and more particularly to thesubstantially real-time determination of velocity profiles of underwatermasses by nonlinear cross-correlation between signal gates or acousticreflection recordings, wherein the correlation variable is velocity.

It is desirable for a number of applications to accurately determine thevelocity of sound propagation through the various acoustic layers in awater mass and in the sea floor and shallow sediments just below thewater floor. For instance, it is possible to ascertain the travel path,and the direction and distance of travel of each ray or segment of wavefront radiated from an acoustic source, if the velocity of each acousticlayer in the water and sediment mass is known.

Large water masses such as the oceans are composed of an extremely largenumber of thin layers each having a velocity differing from the adjacentlayer by a small amount. It has, however, been found that it is possibleto group these thin layers into zones whose overall average velocity isa composite of the individual thin layer velocities. The predictedpropagation path and overall travel time of an acoustic wave throughthese zones will vary by an insignificant amount from the actualobserved path and travel time of the acoustic wave.

It is known that the velocity of sound in sea water is a function oftemperature, depth and salinity. Thus, it has heretofore been known todetermine the velocity of a salt water mass by measuring the temperatureat predetermined depths by means of a thermometer and also ascertainingthe salinity from a sample of water obtained at the same depth. A moremodern technique is to use a device commonly termed a "singaround"velocimeter, such as the velocimeter disclosed in Review of ScientificInstruments, November, I947, Volume 28, No. ll, pages 897-90l, byGreenspan et al. Such singaround" velocimeters provide a continuousmeasure of the acoustic velocity of each small layer of the watertraversed as the velocimeter is lowered or raised through the watermass.

While prior techniques have provided reasonably satisfactory results indetermining the velocity of water at a specific point of measurement,the velocity data thus collected is only applicable at the point ofmeasurement, and the measurements must be taken in a time-consumingvertical point-topoint manner. Because of the point-to-point, widelyspaced apart discrete measurements resulting from prior techniques,velocity-depth profiles heretofore ascertained have often failed toaccurately acoustically describe water layers. For example, theemperical grouping of spaced-apart discrete velocity measurements intosignificant acoustic zones have been found to introduce seriousdiscrepancies between the computed and actual propagation paths ofacoustic signals.

Also, prior velocity measurements have generally been widely spacedapart in a horizontal plane, thus causing discrepancies and errors whenthe water mass is emperically broken down into arbitrarily chosenvelocity layers in dependence upon the widely separated measurementpoints. Prior pointto-point velocity measurements have been extremelytime consuming. It has been found that acoustic layering changes dailyin the ocean mass in an unpredictable manner, and thus such discretemeasurements taken over a period of time provide additional ambiguitiesinto the final acoustic profile. Further, with prior art velocitytechniques, the velocity-relationships are obtained only after alaborious process in interpretation of the data. Thus, final results arenot available until days and weeks after the collection of the velocitydata.

In accordance with the present invention, a method and apparatus isprovided for continuously and accurately dividing an underwater massinto significant acoustic layers and determining the velocity of eachlayer in a substantially real-time manner. In the present invention,generally instantaneous determination is provided for significantvelocity data along a continuous horizontal traverse, thus eliminatingthe majority of the problems encountered in prior art techniques, andresulting in immediately available velocity profiling for use indevelopment of ocean resources, national defense and the like.

in accordance with the invention, acoustic waves are sequentiallygenerated along a marine traverse. Signals are generated in response todetection of reflections of the waves at a plurality of detectinglocations along the marine traverse. Indications of the acousticvelocity between underwater reflecting horizons are then generated inresponse to crosscorrelation of selected portions of the signals.

in accordance with a more specific aspect of the invention, acousticwaves are sequentially generated along a marine traverse at spacedintervals such that common depth point coverage is provided ofreflecting horizons within the water layer. Reflections of the acousticwaves from the reflecting horizons are received at a plurality oflocations in order to produce signals. Signal gates of selected ones ofthe signals are cross-correlated wherein acoustic velocity is thecorrelation variable. Composite velocity correlation functions areproduced in response to the cross-correlation products, the peakpositive point of the composite functions providing indications of theaverage velocities of the water masses through which the acoustic waveshave traveled. From the average velocities, the sectional velocity ofthe intervals between the underwater reflecting horizons is determinedon a substantially real-time basis.

For a more complete understanding of the present invention and forfurther objects and advantages thereof, reference is now made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 somewhat diagrammatically illustrates an embodiment of theinvention;

FIG. 2 is a block diagram of the component parts of the preferredembodiment of the invention; and

FIG. 3 is a diagrammatic illustration of the depths and travel timesutilized by the invention.

Referring to FIG. 1, a marine vessel 10 is illustrated as streamingthree hydrophone detector systems l2, l4 and 16 along a marine traverseby means of the cable 18. The hydrophone detector systems 12-16 maycomprise any suitable type conventionally used, as for example, thesystems disclosed in US Pat. No. 3,386,526, issued June 4, l968 toKenneth E. Burg. While three hydrophone detector systems have beenillustrated, it will be understood that more or less systems may beutilized to provide different desired operating characteristics. Themarine vessel 10 also tows therebehind an acoustic wave source 20.Source 20 may comprise any one of a number of conventional acoustic wavesources, such as an air gun, a gas explosion device, dynamite shots oran acoustical radiating antenna such as disclosed in US. Pat. No.3,334,328, issued to Kenneth E. Burg et al.

Source 20 is sequentially energized during a marine traverse from vessel10 to generate acoustical waves which travel downwardly and which arereflected upwardly for reception by the hydrophone detector systems l2,l4 and 16. For instance, certain of the acoustic waves travel raypaths22ac and are reflected upwardly from water horizon or interface 24. Asis well known, the angle of incidence of the acoustic waves is equal tothe angle of reflection. The average velocity of the water intervalbetween the water surface and the interface 24 is designated as V,.

In a similar manner, acoustic wave energy passes through the interface24 and is reflected via raypaths 26a-c from a water interface 28 forreception by the hydrophone detector systems. Acoustic energy alsotravels the deeper raypaths 300-0 for reflection from the interface 32.Interface 32 may comprise a water-reflecting horizon interface, or maycomprise the sea floor or shallow sediments just below the sea floor.The average acoustic velocity between the water surface and interface 28is designated as V,, while the average sectional acoustic velocitybetween the interfaces 24 and 28 is designated as V The average acousticvelocity between the water surface and interface 32 is denoted as V,,with the average sectional acoustic velocity between interfaces 28 and32 being designated as V,,. The depths Z,-Z, and travel times T T, ofthe acoustic waves for reflecting horizons 24, 28, and 32 areillustrated in FIG. 3.

The acoustic wave source 20 is sequentially energized along the marinetraverse at time intervals related to the time consumed by the vessel intraveling a distance equal to onehalf the distance between thehydrophone detector systems l2, l4 and 16. The effect of thissynchronization of the source with the distance of travel of the vessel10 is to superimpose the reflection points at each underwater reflectinghorizon for progressively greater distances between the source 20 andthe receiving hydrophone detector. This technique is commonly termed"common depth point coverage, and is disclosed in a number ofpublications including Geophysics, Volume XXV, No. 6, Part ii, Dec.i962, by W. Harry Mayne; in U.S. Pat. No. 2,732,906, issued Jan. 31,1956 to W. Harry Mayne, and U.S. Pat. No. 3,217,828, issued Nov. 16,1965 to Mendenhall et al. As will be later described, the common depthpoint coverage according to the invention eliminates the introduction ofambiguities into the determination of velocity because of dip angle ofunderwater interfaces.

Upon detecting reflections of acoustic waves generated by the source 20,the hydrophone detector systems 12-16 generate electrical signals whichare fed through conductors in the cable 18 and through a plurality ofindividual conductors 34 to a recording system 36. The recording system36 may comprise any suitable type of electrical signal recorder, such asa magnetic drum 38 mounted on a shaft 40 and driven by a motor 42. Aplurality of recording heads 44 store the electrical signals as separateacoustic data traces on the drum 38. The stored signals may bereproduced by playback heads 46. The acoustic data traces may be storedon drum 38 in analog form. Alternatively, the traces may be sampled andrecorded on magnetic tape as a conventional digital acoustic record inthe manner described in U.S. Pat. No. 3,134,957 issued to Foote et al.

The acoustic signal recorded on a selected one of the traces is appliedvia a channel 48 to one input ofa correlation system 50. A signal fromanother selected trace on the drum 38 is applied through a normalmoveout unit 52 to another input of the correlation system 50. Acorrelation system output is applied via a conductor 54 through a switcharm 56 to any one of a plurality of recording heads 58 on a multitrackrecording drum 60. Drum 60 is driven by a motor 62. Switch arm 56 may beselectively actuated to apply the correlation signal output to any oneof the traces l5 on drum 60. Playback heads 64 connect the traces 1-5 onthe drum to a summation unit 66, the output of which is applied via aconductor 68 to a recording head 70, so that the summation signal may bestored upon the drum 60.

Selected ones of the outputs from the recording heads 44 are fed into aninput of a time interval detector circuit 72, along with indications ofthe time of energization of source 20. Circuit 72 detects the timeinterval between the generation of acoustic waves from source 20 to thereception of the waves at the hydrophone detector systems. Circuit 72then divides the detected time interval to generate indications of thetravel time of an acoustic wave down to selected water interfaces.Circuit 72 may comprise, for instance, a bistable multivibrator circuitwith suitable output voltage divider circuitry.

The output from circuit 72 is fed to a multiplier 74, which multipliesthe travel time by the average velocity of a selected water mass whichis fed from playback head 64 via lead 76. As will be later described,the output from multiplier 74 is representative of the depth to theselected interface. The time and depth signals for selected interfacesare fed to operational circuit 78, which may comprise a special purposecomputer. Circuit 78 generates indications of the average sectionalacoustic velocity which may be recorded on any suitable dis-- play orprofiler.

The crosscorrelation portion of the processing system thus described ismore fully disclosed in U.S. Pat. No. 3,417,370. issued to Brey on Dec.17, 1968. The Brey patent discloses the operation of the processingsystem to determine the average acoustic velocity from an acousticsource to a reflection point. The disclosure of the system and operationcontained within the l'lrcv pulcltl is hereby incorporated herein.Basically, the

Brey patent determines acoustic velocity by utilizing unique propertieswhich permit corrections for normal moveout. More specifically, thevelocity down to a selected reflector is obtained by utilizing tworelated reflection traces from the reflector by effectively correctingone of the traces for normal moveout over a given signal gate whichincludes a reflection from the selected reflector. This normal moveoutcorrection is based upon each of a plurality of different selectedvelocities.

The corrected data gate is cross-correlated with the uncorrected secondtrace having a different geometry, wherein the variable in thecross-correlation is velocity. The resultant cross-correlation functionwill have a positive maximum peak which is representative of the actualvelocity through which the reflection has passed. This operation iscarried out for each of a plurality of pairs of the acoustic data tracesand for each of a plurality of time gates located at successivelyincreasing record times. The velocity correlation functions of thedifferent pairs of the traces are summed, as by stacking, to providehigh resolution.

Referring to FIG. 1, a window or signal gate is taken on one of thetraces recorded on the drum 38 and the normal moveout correction unit 52shifts the events within the signal gate nonlinearly in the well knownnormal moveout correction manner. The nonlinearly shifted signal gate isthen cross-correlated with zero time delay and the cross-correlationsystem with a signal gate from the trace sensed by the conductor 48. Theamplitude from this cross-correlation is then registered or stored ontrack I of the drum 60,

Drum may be rotated or moved under control of the stepping motor 60 forincrementing the drum relative to the recording heads 58 for eachselected normal moveout cyclc Each of the traces stored on the drum 3!!have time gates thereof which are corrected for normal movcout for agiven velocity, or cross-correlated with an appropriate time gatc fromthe reference signal trace, and then recorded on sub sequent channels onthe drum 60. The cross-coirclation products are then summed by thesumming circuit ()1) and recorded by the recording head 70. Morespecifically, thc present method is accomplished by taking a velocitycorrelation between any and all pairs of traces flt) and g(t) on theacoustic reflection recording a velocity correlation is obtained masAverage velocity V is then chosen as the V, which results in the maximumpositive value of (V,). It should be noted here that the velocitycorrelations which are stacked to form the function wt V are not limitedto all being from the same record. A spatial average from more than onerecord will tend to reduce be expressed as a function of vertical traveltime T,

2X -T n 7.2 T 5) l P Again as r1 U.

X i r .I

wherein T,.= vertical travel time to the selected reflector.

In any case for single fold operations the clip angle (a) is assumed tobe known and V is determined as outlined above. The Tfs are made up fromthe gate centers, i.e.,

as specified for the cth offset traces [f(t)] in the above N velocitycorrelations and also from the moveout times corresponding to these gatecenters as determined by using V for the ith offset traces [g(t)] in theN velocity correlations. Thus, from the N velocity correlations whichproduce M V there are 2N T,- 5 determined, some of which are redundantif the same trace is used more than once in the N correlations.

It remains to solve equation 5 for T, and this may be done by knownmethods including the method of at least squares with the knownconstraints on V and a,

where In the special case where rr U.

The use of the common depth point coverage of the invention eliminatesany required knowledge of the dip angle a. In accordance with theinvention, after the average velocity from the water surface to thefirst reflector 24 is determined, later time gates are taken of therecorded acoustic signals in order to determine the average velocity ofthe water mass from the waters surface to lower water-reflectinghorizons such as 28 and 32. The same step of correcting these time gatesfor normal moveout, cross-correlating and summing the cross-correlationfunctions for each ofa plurality of assumed velocities will be thencarried out for these later time gates. Thus, the average velocities V,,V and V are determined by the invention.

An important aspect of the invention is the determination of the averagesectional velocity V between the reflecting horizons 24 and 28 and thedetermination of the average sectional velocity I between the reflectinghorizons 28 and 32. This is accomplished by the circuits 72-78 accordingto the following equations:

Z,=T,V,

(8) a a a wherein. according to FIG. 3,

Z,=depth to reflecting horizon 24 Z =depth to reflecting horizon 28 Zdepth to reflecting horizon 32 T,=travel time of acoustic wave tointerface 24 T =travel time of acoustic wave to interface 28 T =traveltime of acoustic wave to interface 32 V,, V and V being previouslydefined.

Hence,

, Z I'm I I I v a L al 7] 3 h 1:. Vs,, Vs. and Vs being the averagesectional velocities as previously defined.

While only three reflecting water interfaces have been used fordescription purposes. it will be understood that the invention will beutilized to compute a large number of such average velocities for greatdepths.

Although the invention has been described with respect to operation ofthe system shown in FIG. I, where acoustic data is recorded in thedigital manner disclosed in the Foote et al. U.S. Pat. No. 3,134,957,normal moveout correction operations may be advantageously carried outby use of a digital computer. When the invention is accomplisheddigitally. the system shown in FIGv 2 will be preferred. The acousticsource 20 and the hydrophone systems l2l6 are illustrated in block form,with a plurality of reflection signals being fed from the hydrophonesystem into a time-varied gain and switching device 80.

Device may comprise any suitable conventional gain control amplifyingsystem, such as for example, the field system sold under the trade nameDFS/l0,000 manufactured and sold by Texas Instruments Incorporated, orthe system identified as PTIOO and sold by the SIE Division of DresserElectronics. Such gain control compensates for variations of signallevel caused by the variations in the overall travel time to the variousacoustic interfaces, in the well-known manner. A switch (not shown) isdisposed in each of the output channels of the hydrophone systems inorder to enable selective samplings of the output from any one of thehydrophones. As will be later described in more detail, such sampling isrequired in order to operate upon reflections from both shal low anddeep reflecting horizons.

The gain-conditioned analog signals from the device 80 are fed to asumming system 82 and also to a time-varied multiplexer andanalog-to-digital converter 84. The multiplexer and converter 84 maycomprise for instance the system disclosed in the Foote et al., U.S.Pat. No. 3,l34,957,

A digital output from the converter 84 is fed to a storage unit 86,which may comprise any suitable magnetic disc or tape storage. Storeddigital data may be fed to an input of the navigation system 88 whichcontrols the operation of the engine and steering controls 90 of thevessel 10. The stored data is fed to the computer 92, which may comprisefor instance, a properly programmed TIAC computer manufactured and soldby Texas Instruments Incorporated, or a CDC3600 computer. The normalmoveout correction and cross-correlation previously described isconducted within the computer 92.

Such cross-correlation of digital signals is well known and is describedin a number of publications. For instance, U.S. Pat. No. 3,075,607,issued Jan. 29, 1963, to Aitken et al., and assigned to the presentassignee, describes digital cross-correlation in detail. Otherdescriptions of typical cross-correlation techniques may be found inGeophysics, Volume 33, No. l Feb. I968), page -126, by Schneider et al;and in the copending patent application Ser. No. 550,3l4, entitled SpaceAveraged Dynamic Correlation Analysis, by Backus et al.

The velocity profile information determined by the computer 92 is fed toa suitable display 94 to provide substantially real-time velocityprofile information. For instance, the display 94 may comprise an X-Yplotter, a cathode-ray tube or the like. Additionally, the velocityprofile information is stored in a storage media 96 for application toperipheral systems such as an undersea warefare system 98. An outputfrom the computer 92 is also fed to the navigation system 88, to providecontrol to the ship's speed and direction. Control signals are fed fromcomputer 92 via channels 100 to the various portions of the system toperform control functions. For instance, the computer 92 controls therate at which the acoustic source 20 is energized. Additionally, thecomputer 92 controls the switching operation provided by the switchingdevice 80.

Acoustic interfaces or reflecting horizons in a water mass vary in depthfrom only a few hundred feet or less up to 20,000 or 30,000 feet. Toachieve the required accuracy for the present system over this widerange of depths, a detector hydrophone system must operate with anonuniform sourceto-detector spacing. An example of such spacing isillustrated in the following table.

TABLE 1 Depth to Acoustic Interface (Feet! Distances Source toHydrophones [Feet] From Table l, it will be seen that data from thenearest detector hydrophone system is utilized to compute the averagevelocity to the shallowest acoustic interfaces, with the more remotehydrophone systems being utilized to compute the velocity to the deeperinterfaces. Similarly, at the deeper distances, data from additionalhydrophones is required. The switching device 80 is utilized to suitablyconnect up the hydrophone system under the control of computer 92 toprovide the desired distance between the source and hydrophonesaccording to the table.

In order to provide common depth point coverage, the time intervalbetween successive impulses of the acoustic source 20 is a function ofthe interval between the hydrophones, the speed ofthe essel 10, and thedepth of the acoustic reflecting horizons. However. it is not generallypossible to achieve common depth point coverage over the wide range ofdepths encountered at practical boat speeds. Thus, the acousticreflections from great depths comprise a sequence of individualreflection patterns superimposed with time intervals equal to theinterval between acoustic source pulses. The computer 92 detects theseindividual superimposed reflection patterns by cross andautocorrelations in a conventional manner.

The delay time between the reflection patterns is compared with the timebetween the acoustic impulses of the source 20, and the reflectionpatterns are then time shifted to permit superposition and stacking ofthese patterns in order to improve the signal-to-noise ratio. As anexample, a vessel traveling at 6 knots moves approximately feet persecond. To arrive at the common depth point coverage for hydrophones 40feet apart as are required to obtain the desired shallow data between 80and 160 feet depths, the acoustic source would be pulsed about every 2seconds. However, the time required for acoustic reflection returns from20,000 feet is approximately 8 seconds, and thus four reflectionpatterns would be provided at 2-second intervals. These reflectionpatterns are autocorrelated, time-shifted and summed by the computer 92in the manner described.

The vessel 10 travels at an essentially constant speed, and the source20 is pulsed at essentially uniform intervals under the control ofcomputer 92 to permit synchronization and summing of the receivedsignals. The time interval between the pulsing of the acoustic source 20is chosen to permit collection of data from the shallowest acousticinterfaces. Thus,

data from the more remote hydrophones for deeper interfaces will oftennot be recorded for each impulse of the acoustic source. The outputs ofthe hydrophones are thus switched under the control of computer 92.

ln order to achieve the required accuracy for each ofthe interface depthranges, a particular relationship exists between the desired frequencyto be recorded and a depth of the acoustic interface. For example, fordepths of l60 feet, the differential travel time, called the normalmoveout, used to determine the vertical velocity in the mannerpreviously disclosed will need to be measured with accuracy of 1/10 to1/100 of a millisecond. At deeper depths of 1,000 feet, the

normal movcout accuracy needed would be of the order of l to 1/10millisecond. Thus, the frequencies required to achieve the same accuracychanges inversely with the depth of the interfaces.

To efl'iciently handle the recording of the data from the gain andswitching device 80, the sampling rate of the system will be changedwith the frequency or with depth. To achieve this, the multiplexer andconverter 84 are under the control of the computer 92 in order to enablevariation of the sample rate. By changing the sample rate, the number ofbits that must be recorded on the storage unit 86 is maintained at anoptimum which is related to the accuracy required and to the depth ofthe acoustic layer involved.

Under some circumstances, it is desirable to sample the data at rateshigher than would ordinarily be needed by the computer 92 to achieve theoperational accuracy. For instance, a high rate of sampling is desirablewhen it is anticipated that the data would be analyzed at a later timeor for another purpose, either by the computer 92 or by shore-basedcomputer. In such case, the data sample is stored on magnetic tape orthe like in the storage 86 so that it would be available for laterprocessing.

Whereas the present invention has been described with respect tospecific embodiments thereof, it is to be understood that variouschanges and modifications will be suggested to one skilled in the art,and it is desired to encompass such changes and modifications as fallwithin the scope of the ap pended claims.

What is claimed is:

1. A system for determining the acoustic velocity profile of water andsediment masses comprising:

a. means for sequentially generating acoustic waves along a marinetraverse at spaced intervals sufficient to provide common depth pointcoverage of reflecting horizons, the generation of said acoustic wavesbeing controlled by a digital computer,

b. hydrophone means for receiving at a plurality of locationsreflections from said acoustic waves to produce signals, said signalsbeing selectively sampled by a digital computer such that signals fromselected horizons can be examined,

. means for cross-correlating selected ones of said signals whereacoustic velocity is the correlation variable.

d. means responsive to said cross-correlation for producing a compositevelocity correlation function the peak positive points of which provideindications of the average velocities of said acoustic waves in thewater and sediment masses through which said acoustic waves havetraveled, and

e. means responsive to said average velocities to determine the averagesectional acoustic velocity profile of selected portions of said waterand sediment masses.

2. The system of claim 5 and further comprising:

a marine vessel streaming a plurality of hydrophones for reception ofsaid reflections.

3. The system of claim 5 and further comprising:

means for switching between selected ones of said hydrophones forreception of reflections from different selected depths.

4. The system of claim 1 wherein said signals are sampled digitally andsaid cross-correlation is accomplished digitally, the rate of saidsampling being varied in dependence upon the depth of the underwaterreflecting horizons being examined.

5. A method of profiling the acoustic velocity of underwater massescomprising:

a. sequentially generating acoustic waves along a marine traverse atspaced intervals sufficient to provide common depth point coverage ofunderwater reflecting horizons,

b. receiving common depth point reflections from said acoustic waves toproduce signals,

c. digitally sampling said signals in dependence upon the underwaterdepth desired to be examined,

d. cross-correlating ones of said signals where acoustic velocity is thecorrelation variable,

e4 producing from said cross-correlation composite velocity correlationfunctions the peak positive points of which provide indications of theaverage velocities of said acoustic waves in the water masses throughwhich said acoustic waves have traveled, and

val.

1. A system for determining the acoustic velocity profile of water andsediment masses comprising: a. means for sequentially generatingacoustic waves along a marine traverse at spaced intervals sufficient toprovide common depth point coverage of reflecting horizons, thegeneration of said acoustic waves being controlled by a digitalcomputer, b. hydrophone means for receiving at a plurality of locationsreflections from said acoustic waves to produce signals, said signalsbeing selectively sampled by a digital computer such that signals fromselected horizons caN be examined, c. means for cross-correlatingselected ones of said signals where acoustic velocity is the correlationvariable, d. means responsive to said cross-correlation for producing acomposite velocity correlation function the peak positive points ofwhich provide indications of the average velocities of said acousticwaves in the water and sediment masses through which said acoustic waveshave traveled, and e. means responsive to said average velocities todetermine the average sectional acoustic velocity profile of selectedportions of said water and sediment masses.
 2. The system of claim 5 andfurther comprising: a marine vessel streaming a plurality of hydrophonesfor reception of said reflections.
 3. The system of claim 5 and furthercomprising: means for switching between selected ones of saidhydrophones for reception of reflections from different selected depths.4. The system of claim 1 wherein said signals are sampled digitally andsaid cross-correlation is accomplished digitally, the rate of saidsampling being varied in dependence upon the depth of the underwaterreflecting horizons being examined.
 5. A method of profiling theacoustic velocity of underwater masses comprising: a. sequentiallygenerating acoustic waves along a marine traverse at spaced intervalssufficient to provide common depth point coverage of underwaterreflecting horizons, b. receiving common depth point reflections fromsaid acoustic waves to produce signals, c. digitally sampling saidsignals in dependence upon the underwater depth desired to be examined,d. cross-correlating ones of said signals where acoustic velocity is thecorrelation variable, e. producing from said cross-correlation compositevelocity correlation functions the peak positive points of which provideindications of the average velocities of said acoustic waves in thewater masses through which said acoustic waves have traveled, and f.operating on selected ones of said average velocities to determine theaverage sectional velocity of the intervals between selected underwaterreflecting horizons.
 6. The method of claim 5 further including signalgates which are related to one another in dependence upon the horizontalspacing between the points of reception of said reflections.
 7. Themethod of claim 5 wherein said step of operating on comprises: dividingthe depth interval of reflecting horizons by the travel time of anacoustic wave through said depth interval.